The invention relates to an optical focusing-error detection device for determining deviations between the plane of imaging of an objective system in an optical apparatus and a radiation-reflecting surface in said apparatus on which surface an image is to be formed. Such a focusing-error detection device comprises a radiation source, the objective system, an astigmatic system comprising two cylindrical lenses arranged in the path of a beam reflected by the reflecting surface, and a radiation-sensitive detector comprising four sub-detectors.
Such a focusing-error detection device may be employed in an apparatus for reading a record carrier having an optically readable radiation-reflecting information structure in order to keep the read beam focused on the information structure.
Such read apparatus is known, inter alia from U.S. Pat. No. 4,025,949. This apparatus is for example employed for reading a record carrier on which a video program is stored. The information structure then comprises a multitude of information areas which, in the track direction, alternate with intermediate areas. The areas may for example be arranged along a spiral track. The information areas have a different influence on the read beam than the intermediate areas. The information may be encoded in the frequency of the information areas and/or in the ratio of the length of said areas relative to that of the intermediate areas. Alternatively the information may be encoded in digital form. Apart from video and audio information, the record carrier may contain digital information, for example, supplied by and to a computer.
For a sufficiently long playing time of the record carrier, when the record carrier has limited dimensions, the details of the information structure will be very small. For example, if a thirty-minute video program is stored on one side of a round disc-shaped record carrier in an angular area having an outer radius of approximately 15 centimeters and an inner radius of approximately 6 centimeters, the width of the tracks will be approximately 0.6 microns and the average length of the information areas will be of the order of one micron.
In order to enable such small details to be read an objective system having a comparatively large numerical aperture must be used. However, the depth of focus of such an objective system is small. Since in the read apparatus the distance between the plane of the information structure and the objective system may vary beyond the range of the depth of focus, steps must be taken in order to detect said variations and to correct the focusing in response thereto.
For this purpose the read beam received from the record carrier may be rendered astigmatic by means of, for example, a cylindrical lens. Between the focal lines of the astigmatic system comprising the objective system and the cylindrical lens a radiation-sensitive detector comprising four sub-detectors may be arranged. If the position of the plane of the information structure relative to the objective system varies, the shape of the image spot formed on the composite detector will change. This change of shape can be detected by appropriately combining the output signals of the sub-detectors.
Such a focusing-error detection device is suitable not only for use in apparatus for reading a radiation-reflecting record carrier, but in general it may be employed in optical apparatus in which it is required to detect deviations between the desired and the actual position of the radiation-reflecting surface onto which an image is to be formed. Examples of this are microscopes, apparatus employed in the fabrication of integrated circuits for projecting a mask onto a substrate, etc.
The focusing-error detection device using one cylindrical lens supplies a signal which is not ideal for control purposes. The characteristic representing the variation of the focusing-error signal as a function of the deviation between the desired and the actual plane of focusing is curved and, in addition, it is asymmetrical around the zero point. The non-linear variation is caused by the different convergence of the beam in two mutually perpendicular directions. The asymmetrical shape of the characteristic is caused by the non-symmetrical position of the images of the pupil of the objective system relative to the plane of the sub-detectors. This asymmetry is particularly serious if the radiation-sensitive detector is disposed at a comparatively large distance from the objective system, which situation may occur in a write or read apparatus in which only the objective system is movable and the other optical elements of the apparatus are stationary.
In U.S. Pat. No. 4,025,949 it is proposed to improve the characteristic of the focusing-error signal by means of two cylindrical lenses of equal power whose cylinder axes are transverse to each other, which lenses are disposed in a parallel beam. This yields a beam having the same convergence in two mutually perpendicular planes, so that the focusing-error signal varies linearly around the zero point. However, now the images of the objective pupil are still non-symmetrical relative to the detector plane, so that the signal remains asymmetrical.